Today more and more ready-to-eat food is sold in grocery stores. Such meals are either frozen or refrigerated in order to have a reasonable long shelf life. Frozen food may be regarded as less tasty and looking less appetizing, while refrigerated food may be stored for a shorter period of time before being eaten. In order to prolong the shelf life for such refrigerated ready-to-eat food pasteurisation is used. Pasteurisation is combined with a removal of oxygen from the interior of the package in order to minimise bacterial growth. Such removal of oxygen may be either of creating a slight vacuum or the replacement of the oxygen by some other suitable gas.
In a cooking and pasteurisation method used by the applicant the food is placed in a plastic tray and is provided with a plastic cover in the form of a see-through thin film which is sealed along the tray edges to create an interior which is completely sealed off from the surroundings. In this plastic film a valve is created which may be automatically opened when an overpressure occurs within the package. Such overpressures are e.g. created when the package is positioned in a microwave oven and the food is cooked by exposing it to electromagnetic radiation. It may also be created through convection in e.g. furnaces with air heating and steam, or by exposing the package to thermal radiation, e.g. through infra-red radiation. When the food is cooked, a large amount of steam is created. The steam is building up an overpressure such that the valve is opening and letting both oxygen and steam out. When the food is cooked the microwave oven is shut off, whereby the steam production instantaneously stops. The valve is then designed to close immediately due to the reduction of internal overpressure and the lowering of the ambient temperatures. The food packages are cooled to a suitable storing temperature and may be delivered to retailers for later use. The overpressure of the kind when the valve opens in such food packages is quite high, in the region of 100 hPa, in comparison to i.a. a valve used in connection with packages such as for coffee, in which the overpressure is just slightly above atmosphere pressure.
The closure of the valve after cooking is important in order to seal the interior of the package and food from ambient air and contaminating substances. The valve must hence be designed such that it is certain that it is not affected by i.a. food residuals or moisture that may be blown into the valve during cooking.
One important closing feature of the valve on the plastic film is the ability to limit the formation of wrinkles in the material and bubbles between the different layers of valve and/or film after cooking and during resealing of the food package. The valve is usually connected to the film by an adhesive, and this adhesive must withstand a quite malicious treatment during the life cycle of the food package. However, so far there is still a too large risk for such formation and the present invention is aiming at further reducing such formation.
Another important feature of the valve is that it must also remain on the plastic film and not tend to fall off during cooking or handling. A reliable fastening of the valve to the plastic film may involve rather complicated, and thereby costly, production processes. It is therefore also an aim of the present invention to provide a valve that may be reliably fastened to the plastic film of the food package in a manner that is less complicated, as compared to previously known valves.